This invention relates to the etching of thin films during the production of devices such as integrated circuits, and more particularly, to the use of solutions containing very dilute fluoride species to etch high-k materials.
Most integrated circuits currently produced are manufactured on thin disks of silicon and/or other semiconductor material (wafers) using xe2x80x9ccomplementary metal oxide semiconductorxe2x80x9d (CMOS) technology. A general discussion of CMOS technology can be found in xe2x80x9cSilicon Processing for the VLSI Era, Volume 2xe2x80x94Process Integrationxe2x80x9d by Wolf et al., Lattice Press, 298-367, (1990). In a CMOS circuit, an electric potential applied to a transistor""s gate electrode capacitively couples to its channel and controls the current that flows between its source and drain electrodes. The gate electrode is electrically insulated from the channel by the gate dielectric. The gate dielectric has historically utilized SiO2 formed by thermally oxidizing the silicon above the channel. SiO2 dielectrics have many advantages, including their ability to be removed by etching in either gas, plasma or liquid based processes.
The electrical properties of the transistor depend to a significant degree upon the nature of the gate dielectric. In particular, reducing the thickness of the dielectric increases the capacitive coupling between the gate and channel, allowing higher speed transistor operation at lower operating voltages. But, as the thickness of the dielectric is reduced much below about 2 nm, quantum tunneling effects tend to increase, allowing an electric current to flow between the gate and channel. This tunneling current is undesirable as it increases the transistor""s power requirements and causes undue heat generation.
Excessive tunneling can be alleviated if the capacitive coupling between the gate and channel is increased by increasing the dielectric constant (k) of a fixed xe2x80x9cphysicalxe2x80x9d thickness of gate dielectric, tphys. The equivalent xe2x80x9celectrical thickness,xe2x80x9d telect, of a high-k gate dielectric is approximately equal to the gate""s physical thickness times the ratio of the dielectric constants of SiO2 and the high-k material, kSiO2 and khigh-k, respectively. That is:
telect=tphys*(kSiO2/khigh-k)
Early approaches to increasing k included xe2x80x9cnitridingxe2x80x9d the SiO2, forming silicon oxy-nitrides (SiOxNy) of various stoichiometries. Recent work evaluating the electrical, metallurgical and chemical properties of various dielectric materials has focused upon the unary oxides of aluminum, zirconium and hafnium, mixtures of these oxides, and silicates of these elements or mixtures. A general discussion on the need for high-k dielectrics can be found in xe2x80x9cHigh-k Gate Dielectric Materials,xe2x80x9d by R. M. Wallace et al., MRS Bulletin, 27 (3), 192-197, (2002). A general discussion on the selection process that leads to particular metal oxides can be found in xe2x80x9cA Thermodynamic Approach to Selecting Alternative Gate Dielectrics,xe2x80x9d by D. G. Schlom, MRS Bulletin, 27 (3), 198-204, (2002).
A subtractive process for producing patterned dielectric features on semiconductor wafers involves depositing a film of high-k material on the wafer, and then etching the film away in areas where it is not desired. While SiO2 is easily etched, the above mentioned high-k materials are highly resistant to chemical attack. Concentrated hydrofluoric acid is the only commonly used etchant for these high-k materials. HF is used, for instance, to reclaim wafers used to test a high-k film deposition process by removing the entire film, allowing their reuse in subsequent tests. In etch testing to date, high-k films that have been annealed (subject to a thermal treatment typical of the CMOS production process) typically etch much more slowly than xe2x80x9cas depositedxe2x80x9d films.
Unfortunately, concentrated HF also has a high etch rate on other silicon-based oxides such as SiO2 formed by thermal oxidation (Tox) of silicon, SiO2 formed by deposition from gas-phase tetraethylorthosilicate and oxygen or ozone (TEOS), and TEOS doped with boron and/or phosphorous (BPTEOS, BTEOS, PTEOS). This means that conventional aqueous HF solutions previously have not been practically useful for selectively etching high-k films relative to silicon oxide materials. Features formed from these silicon oxides are critical to the circuit""s operation and are often necessarily present on the wafer when a high-k film is etched away. If high-k films are to be commercially viable, it is highly desired that an etching chemistry be found that can selectively etch a high-k film, with little or no etch of the other films present. Etch selectivities of at least about 1:1, more preferably greater than about 3:1, and more preferably greater than about 5:1 are desired. Silicon oxides typically etch faster than these high-k films in concentrated HF, with selectivities of high-k film to silicon oxide being in a range from 1:10 to even 1:100.
The situation is somewhat similar to that of selectively etching silicon nitride (Si3N4) in the presence of SiO2. Deckert initially investigated selective etching of Si3N4 in aqueous fluoride media, but was unable to achieve the desired etch selectivities (C. A. Deckert, xe2x80x9cEtching of CVD Si3N4 in Acidic Fluoride Media,xe2x80x9d J. Electrochemical Soc., 125 (2), 320-323 (1978)). Deckert then pursued etching in HF/organic solvent mixtures and was able to achieve a 2.5:1 selectivity with 2 wt % HF in glycerol at 110xc2x0 C. (C. A. Deckert, xe2x80x9cPattern Etching of CVD Si3N4/SiO2 Composites in HF/Glycerol Mixtures,xe2x80x9d J. Electrochemical Soc., 127 (11), 233-2438 (1980) and U.S. Pat. No. 4,269,654 to Deckart). U.S. Pat. No. 4,269,654 also noted that 2 wt % HF in water at 96xc2x0 C. gave a very poor, 1:4.5 etch selectivity of Si3N4 to SiO2. Jagannathan and Rath later used HF/organic mixtures for the selective removal of SiO2 without attack of coexisting metals (U.S. Pat. No. 6,200,891 to Jagannathan and U.S. Pat. No. 6,254,796 to Rath). Itano et al. has demonstrated the selective etching of unannealed HfSiOx and ZrAlOx films over thermally grown SiO2 in mixtures of HF in a low dielectric constant solvent (for example, isopropyl alcohol, acetic acid, tetrahydrofuran, methanol or ethanol), (M. Itano et al., xe2x80x9cSelective and Non-Selective Wet Etching by Low-Relative Dielectric Constant Solvent Containing Fluorine Compound,xe2x80x9d presented at the International Sematech Wafer Clean and Surface Preparation Workshop, May 21, 2002, Sematech International, Austin, Tex.).
The chemistry of aqueous HF solutions and its etch mechanisms have been investigated by Verhaverbeke and Knotter (S. Verhaverbeke et al., xe2x80x9cThe Etching Mechanisms of SiO2 in Hydrofluoric Acid,xe2x80x9d J. Electrochemical Soc., 141 (10), 2852-2852, (1994); D. Martin Knotter, xe2x80x9cEtching Mechanisms of Vitreous Silicon Dioxide in HF-Based Solutions,xe2x80x9d J. Am. Chem. Soc., 122 (18), 4345-4351, (2000)). Verhaverbeke and Knotter noted that the concentration of the various ionic species in aqueous HF (e.g., HF, HF2xe2x88x92, H2F2, H+, Fxe2x88x92) vary with HF concentration, ascribing the etching of SiO2 to HF2xe2x88x92 and H2F2. Knotter (2001) further investigated the etching mechanisms of Si3N4 in aqueous HF.
U.S. Pat. No. 5,382,296 to Anttila notes that aqueous HF concentrations of 0.000049 wt % to 0.049 wt % can be used near ambient temperature to clean xe2x80x9cparticles . . . as well as metallic and organic contaminationxe2x80x9d where the equivalent thickness of the metallic film is approximately 10xe2x88x926 A (Angstrom=10xe2x88x9210 meter), far below one monolayer of film coverage. U.S. Pat. No. 6,300,202 to Hobbs notes that xe2x80x9cmetal oxide dielectrics are not readily susceptible to wet etch processingxe2x80x9d and proposes removing the metal-oxides by first reducing the metal-oxides to metals by annealing in a low-oxygen or hydrogen-rich environment, followed by etching the metallic metals with a wet or dry etch.
In recently published work, Chambers tested the etching of unannealed and annealed films in ambient temperature, 0.49 wt % aqueous HF, and concluded that for annealed films xe2x80x9cSelectivities of metal silicate to silicon dioxide of  less than 0.1 were measured, indicating the need for alternative high-k wet etch chemistries with high selectivity to silicon dioxide and siliconxe2x80x9d (J. J. Chambers et al., xe2x80x9cEffect of Composition and Post-Deposition Annealing on the Etch Rate of Hafnium and Zirconium Silicates in Dilute HF,xe2x80x9d Proc. 7th Intl. Symp. on Cleaning Technology in Silicon Device Mfg., Electrochemical Society, Pennington, N.J., PV 2001-26, 359).
What is needed, therefore, are etching compositions and methods that provide the desired stripping rate of high-k films present on the wafer, and more preferably the desired selectivity between the high-k films and other films present on the wafer, particularly SiO2. Such compositions desirably would be effective in current semiconductor processing equipment without involving excessive cost or safety concerns.
The present invention provides technology, including processes, equipment, etching solutions, and objects fabricated thereby, applicable to etching high-k dielectric material(s) in an effective, controllable, and repeatable manner. In preferred embodiments, the principles of the present invention may be used to selectively etch high-k dielectric material(s) relative to co-existing materials that might also be present on a substrate such as a semiconductor wafer. The etching composition of the present invention is an aqueous solution that comprises an unconventionally dilute concentration of one or more fluoride species. The composition preferably is used above ambient temperatures to increase the etch rate of the high-k material(s) and, in some embodiments, to further increase the etch selectivity for high-k material relative to other co-existing materials. As an option, etch rate and/or selectivity may be controlled by adjusting the pH of the etching composition. As a further option, both temperature and pH may be used to control etch rate and/or selectivity if desired. After etching, the etched objects, e.g., semiconductor wafers, can be rinsed, dried or otherwise processed using conventional semiconductor processing techniques.
The invention can be practiced using tool sets common to the semiconductor production industry. In semiconductor production, the controlled etching of films in aqueous chemistries is a highly developed technology. Hardware and software are already available to precisely blend a solution, and thereafter to monitor and control its temperature, flow rate, concentration, pH and other solution characteristics as needed. These controls are adequate to consistently provide an etching process with repeatable, controllable results.
In one aspect, the present invention relates to a method for etching one or more microelectronic substrates, each independently comprising a dielectric material having a dielectric constant greater than about 4. The substrate is contacted with an aqueous etching solution comprising at least one fluoride species. The total fluoride concentration in the etching solution is no greater than about 0.2 weight percent.
In another aspect, the present invention relates to an aqueous etching composition. The composition comprises at least one fluoride species, wherein the total fluoride concentration in the etching solution is no greater than about 0.2 weight percent.
In another aspect, the present invention relates to a method for selectively etching a high k dielectric material relative to a co-existing material. A substrate is provided that comprises the high k dielectric material and the co-existing material. An aqueous etching composition is caused to contact the substrate. The composition comprises at least one fluoride species, wherein the total fluoride concentration in the etching solution is sufficiently dilute such that the composition etches the high k dielectric material relative to the co-existing material with a selectivity ratio of greater than about 1:1.
In another aspect, the present invention relates to an apparatus for etching one or more substrates. The apparatus comprises a chamber in which at least one substrate is positioned to carry out an etching treatment. The chamber is fluidly coupled to one or more sources of one or more ingredients that in combination constitute an aqueous etching composition. The ingredients may be combined either before or after being dispensed into the chamber. The composition comprises at least one fluoride species, wherein the total fluoride concentration in the etching solution is no greater than about 0.2 weight percent. A control system comprises functionality to at least help control the etching treatment in a manner effective to cause the aqueous etching composition to contact the at least one substrate during at least a portion of the etching treatment.